Liquid crystal display device

ABSTRACT

A liquid crystal display device including opposite first and second substrates, and column spacers between the first and second substrates, the column spacers formed such that the column spacers differ in size as positions of the column spacers differ, thereby accommodating the variation in gap between substrates when liquid crystal is concentrated at a lower end of the display due to gravity when a large LCD panel is in a vertical position.

This application claims the benefit of the Korean Application No.P2002-78823 filed on Dec. 11, 2002, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND. OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, andmore particularly, to a liquid crystal display device, with a gapsupporting part that prevents defects caused by gravity.

2. Discussion of the Related Art

With the development of an information oriented society, the demands ondisplay devices have increased. To meet the demands, different flatdisplay devices have recently been developed for use in various devices,such as the Liquid Crystal Display Device (LCD), the Plasma DisplayPanel (PDP), the Electro Luminescent Display (ELD), and the VacuumFluorescent Display (VFD).

Among the flat display devices, LCD devices have been used the mostwidely as portable display devices replacing the Cathode Ray Tube (CRT)because LCD devices have excellent picture quality, are light weight,are thin, and have low power consumption. In addition to portable LCDdevices, LCD devices are under development for televisions and computermonitors.

The LCD device has a liquid crystal panel for displaying a picture and adriver that provides a drive signal for the liquid crystal panel. Theliquid crystal panel has a lower substrate and an upper substrateopposite to each other with a gap between the substrates and a liquidcrystal layer between the two substrates. The liquid crystal paneldisplays an image by controlling the light transmittivity of the liquidcrystal with an electric field formed between the two substrates.

The structure and operation of a related art liquid crystal panel in anLCD device will be described briefly with reference to FIG. 1. In FIG.1, the liquid crystal panel has a lower substrate 22 bonded to an uppersubstrate 5 with a gap between the substrates and a liquid crystal layer14 injected between the lower substrate 22 and the upper substrate 5.

The lower substrate 22 has a plurality of gate lines 13 arranged in onedirection at fixed intervals and a plurality of data lines 15 arrangedin a direction substantially perpendicular to the gate lines at fixedintervals that define a pixel regions P. The lower substrate 22 has aplurality of pixel electrodes 17 in pixel regions P defined by the gatelines and the data lines, and a plurality of thin film transistors T areformed at the cross parts of the gate lines 13 and the data lines 15.

The upper substrate 5 has a black matrix layer 6 for shielding the thinfilm transistors T, gate lines 13, and data lines 15 from light. Theupper substrate also has a R, G, B color filter layer 8 for displayingcolors and a common electrode 18 for implementing a picture.

The thin film transistor T has a gate electrode projecting from the gateline 13, a gate insulating film (not shown) formed on the entiresurface, an active layer on the gate insulating film over the gateelectrode, a source electrode projecting from the data line 15, and adrain electrode arranged opposite to the source electrode. The pixelelectrode 17 is formed of a transparent conductive metal such asindium-tin-oxide (ITO).

The LCD device can display a picture by controlling the amount of lightpassing through the liquid crystal layer 14 by changing the orientationof the liquid crystal layer 14. The orientation of the liquid crystallayer at the pixel electrode 17 is set according to a signal from thethin film transistor T.

A related art method for fabricating the liquid crystal panel will bedescribed, briefly. FIG. 2 illustrates a flow chart of the steps for arelated art method of fabricating a liquid crystal panel.

A plurality of gate lines with gate electrodes are formed in onedirection on a panel region of a large sized glass substrate, which isused for forming a plurality of panels. Next, a gate insulating film isdeposited on an entire surface, an active layer is formed on the gateinsulating film over the gate electrode, and a plurality of data linesare formed in a direction substantially perpendicular to the gate linessuch that source and drain electrodes are disposed on opposite ends ofthe active layer. Then, a protection film is formed on the entiresurface, a contact hole to the drain electrode is formed, and a pixelelectrode is formed in the pixel region, to form a thin film transistorarray (st1).

A polymer thin film is deposited on the substrate with the thin filmtransistor array and aligned by rubbing or optical alignment (st2). Therubbing or optical alignment process fixes the initial orientation ofthe liquid crystal, enables regular driving of the liquid crystal, andprovides uniform display performance. In general, the alignment film istypically formed of an organic polymide group.

A seal pattern for bonding the substrates is printed on the periphery ofeach panel region of the glass substrate having the alignment filmformed thereon (st3). The seal pattern serves to form the gap requiredfor injection of the liquid crystal and prevents leakage of the liquidcrystal injected therein. The printing of the seal pattern is a processof screen printing a desired fixed pattern of a thermo-setting resin.

Then, spacers are spread uniformly on the substrate to maintain a fixedcell gap (st4). Fixed size spacers are used to maintain an accurate andconsistent gap between the substrates. The spaces should be uniformlydistributed on the lower substrate. The spacers may be spread by a wetspreading method in which the spacers are spread mixed with alcohol orby a dry spreading method in which only the spacers are spread. Twoexamples of dry spreading are an electrostatic method in which staticelectricity is used and an antistatic method in which a gas pressure isused. The antistatic method is mostly used for liquid crystal panelshaving a structure vulnerable to static electricity.

A black matrix layer is formed on another glass substrate, which is tobe bonded with a substrate having the thin film transistors formedthereon. The black matrix shields portions of the substrate outside ofthe pixel region. Next, the R, G, B color filter layer is formed on eachpixel region, and a common electrode is formed on the entire surface.Then, an alignment film is formed over the common electrode.

When the spacer spreading is finished, the substrate having the thinfilm transistor array formed thereon and the substrate having the colorfilter array formed thereon are bonded (st5). The method of bonding thethin film transistor array substrate and the color filter arraysubstrate depends on the gap tolerance which typically is a fewmicrometers. A bonding error may allow light leakage which will degradethe picture quality of the liquid crystal cell.

Because the bonded substrates have a plurality of panels, the bondedsubstrates are cut into unit panels (st6). The cell is cut by scribing acutting line on a substrate surface with a pen of diamond havinghardness greater than the glass substrate and breaking the bondedsubstrates along the cutting line by applying a force.

Next, liquid crystal is injected into the space between the bondedsubstrates, and the liquid crystal injection hole is sealed (st7). Aliquid crystal panel has a few hundred square centimeters of area and agap of a few micrometers. Therefore, to effectively inject the liquidcrystal into such a panel, a vacuum injection method is used in which apressure difference between inside/outside of the panel forces liquidcrystal into the gap.

The methods for spreading spacers described above have limitations.Consequently, there has been much research on methods of forming spacerpatterns during the fabrication of the substrate. One method suggestscreating a pattern of spacers made of an organic material on an uppersubstrate.

FIGS. 3A to 3F illustrate cross-sections of a panel showing the steps ofa related art method of forming a spacer pattern on a color filter arraysubstrate.

Referring to FIG. 3A, a light shielding metal is deposited on atransparent insulating substrate 5 and portions are removed to form ablack matrix layer 6 to shield light from portions of the liquid crystalpanel outside the pixel regions. The black matrix layer 6 may be a thinmetal film of chrome Cr having an optical density greater than 3.5, afilm of organic material or a bi-layered black matrix of chromeCr/chrome oxide CrOx. Accordingly, the black matrix is formed of one ofabove materials depending on-the application.

Referring to FIG. 3B, the color filter layer 8 a, 8 b, 8 c is formed onrespective pixel regions, of R, G, B color resins. The main ingredientsof the color resin are a photopolymerization type photosensitivecomposition of a photopolymerization initiator, a monomer, a binder, andorganic pigments having colors of red/green/blue. A coat of the redcolor resin is formed on the entire surface of the substrate 5 havingthe black matrix formed thereon and removed selectively to form a redcolor filter layer 8 a in the desired region. Next, a coat of a greencolor resin is formed on the entire surface of the substrate 5 havingthe red color filter layer 8 a formed thereon and removed selectively toform a green color filter layer 8 b in the desired region. Finally, acoat of blue color resin is formed on the entire surface of thesubstrate 5 having the red and green color filter layers 8 a and 8 bformed thereon and removed selectively to form a blue color filter layer8 c in the desired region.

Referring to FIG. 3C, a coat of transparent resin with an insulatingproperty is formed on the substrate 5 to form an overcoat layer 26. Thecoat of transparent resin forms a flat surface over the color filterlayers 8 a, 8 b and 8 c.

In FIG. 3D, a common electrode 18 of a transparent metal, such asindium-tin-oxide (ITO) and indium-zinc-oxide (IZO), is formed on theovercoat layer. A common voltage will flow through the common electrode18 to drive the liquid crystal 14 together with a pixel voltage thatwill flow through the pixel electrode 17 on the thin film transistorarray substrate 22.

Next, in FIG. 3E, a transparent organic film is formed on the entiresurface of the substrate 5 having the common electrode 18 formed thereonand subjected to photolithography and etching to form spacers 20 ofrequired heights.

Referring to FIG. 3F, after the spacers 20 are formed, the entiresurface of the substrate 5 is coated with a transparent organicinsulating material such as polyimide to form an alignment film 22.Then, the surface of the alignment film 22 is rubbed in a predetermineddirection. Thus, a related art color filter array substrate can befabricated.

Alternatively the spacers 20 may be are patterned after the alignmentfilm 22 is formed, but the alignment film 22 under the spacers 20 may bedamaged by chemical used for patterning the spacers 20. Therefore, thealignment film 22 typically is formed after the spacers 20 arepatterned.

FIG. 4 schematically illustrates the cross-section of a bonded thin filmtransistor array substrate and color filter array substrate with spacersformed thereon in a vertical position according to the related art. LCDdevices are mostly employed as monitors, which are used in a verticalposition when the LCD devices are in notebook computers or general LCDmonitors. Also, during testing the LCD panel is placed in a verticalposition. When the LCD panel is vertical the liquid crystal concentratesin the lower end due to gravity resulting in a difference in the liquidcrystal concentration in the upper end and the lower end of the LCDpanel. The higher liquid crystal concentration at the lower end causesthe cell gap between the upper substrate 5 and the lower substrate 22 tovary from top to bottom which results in the cell gap between thesubstrates to become greater than the height of the column spacers 20 atthe lower end of the LCD panel.

The uneven cell gap between the substrates and the separation of thecolumn spacers 20 from the substrate in the lower side causes thepicture quality to differ between the upper end and the lower end. Thisproblem increases as the size of the LCD panel increases.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a liquid crystaldisplay device that substantially obviates one or more of the problemsdue to limitations and disadvantages of the related art.

An advantage of the present invention is to provide a liquid crystaldisplay device with spacers that can adapt to the gap between substrateswhen liquid crystal is concentrated in the lower end due to gravity whena large LCD panel is placed in a vertical position. This will improvepicture quality.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the liquidcrystal display device includes opposite first and second substrates,and spacers between the first and second substrates, the spacers formedsuch that the spacers differ as positions of the spacers differ. Thespacers in the lower end are larger in diameter and thickness than thespacers at the upper end when the liquid crystal panel is in a verticalposition.

In addition, the spacers may have two or more different thicknesses. Thespacers may be formed of photo. The spacers may be formed by an ink jet.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE. DRAWINGS.

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention:

In the drawings:

FIG. 1 illustrates a related art liquid crystal panel, schematically;

FIG. 2 illustrates a flow chart showing the steps of a related artmethod for fabricating a liquid crystal panel;

FIGS. 3A to 3F illustrate cross-sections of a panel showing the steps ofa related art method of forming a spacer pattern on a color filter arraysubstrate;

FIG. 4 illustrates the cross-section of a related art LCD panel, placedin a vertical position, schematically;

FIG. 5 illustrates a plan view showing a column spacer formation on aliquid crystal panel of an LCD device in accordance with a firstpreferred embodiment of the present invention;

FIG. 6 illustrates a side view of the substrate in FIG. 5 having thecolumn spacers formed thereon;

FIG. 7 illustrates an enlarged section of the column spacers placed atthe upper end of the LCD panel in accordance with a first preferredembodiment of the present invention;

FIG. 8 illustrates an enlarged section of the column spacers placed atthe lower end of the LCD panel in accordance with a first preferredembodiment of the present invention;

FIG. 9 illustrates a cross-section showing an LCD panel in a verticalposition, in which column spacers are placed having different sizes inaccordance with a first preferred embodiment of the present invention;and

FIG. 10 illustrates a side cross-section of an LCD device withsubstrates and column spacers in accordance with a second preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. FIG. 5 illustrates a plan view showing a column spacerformation on a liquid crystal panel of an LCD device in accordance witha first preferred embodiment of the present invention, and FIG. 6illustrates a side view of the substrate in FIG. 5 having the columnspacers formed thereon.

The column spacers 200 may be formed on a lower substrate 22 having athin film transistor array formed thereon, or on an upper substrate 5having a color filter array formed thereon. The embodiment shows columnspacers 200 formed on the lower substrate 22.

Referring to FIG. 5, there may be small diameter column spacers 201 onthe surface of the upper end of the lower substrate 22 and largediameter column spacers 202 on the surface of the lower end having adiameter more or less larger than the diameter of the column spacers 201on the upper end. The column spacers 200 may be sprayed from a nozzle ofan ink jet head. While, the use of the ink jet head to spay columnspacers 200 of different sizes is disclosed, the ink jet head may alsobe used to spay column spaces 200 of substantially uniform sizeaccording to the present invention.

For convenience in the description, when the upper substrate 5 and thelower substrate 22 are bonded and in a vertical position the upper sideof the LCD panel is called an upper end, and the lower side of the LCDpanel is called a lower end.

Referring to FIG. 6, the column spacers 200 sprayed from the ink jethead differ; column spacers 201 on the upper end have lower heights andsmaller diameters and column spacers 202 on the lower end have higherheights and larger diameters.

FIGS. 7 and 8 respectively illustrate the column spacers 201 and 202 inaccordance with a first preferred embodiment of the present invention.The column spacers 201 in FIG. 7, illustrate an enlarged view of thecolumn spacer 201 in the upper end of the lower substrate 22. The space201 has a height for maintaining a cell gap between the lower substrate22 and the upper substrate 5.

On the other hand, the column spacer 202 in FIG. 8 has a larger diameterand a higher height than the column spacer 201 in FIG. 7. The columnspacer 202 is placed on the lower end of the substrate 22. The height ofthe column spacer 202 is set according to the expansion ratio of thecell gap between the lower substrate 22 and the upper substrate 5 due togravity. The diameter or the height of the column spacer can becontrolled by the ink jet head.

The gap expands from the upper side to the lower side. For an example,when the height of the column spacer 201 in the upper end is 3.5 μm, theheight of the column spacer 202 in the lower part may be approx. 4 to4.5 μm. Thus, when the difference of cell gaps between the upper end andthe lower end is below 1 μm, it is preferable that the column spacersare sprayed taking the change in the cell gap into account.

FIG. 9 illustrates schematically an LCD panel having the column spacers200 sprayed, two substrates bonded, and liquid crystal injected betweenthe two substrates in accordance with a first preferred embodiment ofthe present invention. When the LCD panel, is in a vertical position,there is a difference in the cell gaps at the upper end and the lowerend. The column spacers 201 and 202 are spray coated so that the heightsand diameters thereof differ to accommodate the difference in the cellgaps.

FIG. 10 illustrates a section of an LCD device having column spacersformed with different heights and diameters in accordance with a secondpreferred embodiment of the present invention. As shown, the columnspacers are distributed in three different areas. The size of the columnspacers is different for an upper end, a middle area, and a lower end.The column spacers 201 sprayed on the upper end are the smallest; thecolumn spacers 202 sprayed at the lower end are the largest; and thecolumn spacers 203 sprayed on the middle area have a size between thesizes of the upper and lower column spacer sizes.

While the first and second embodiments described herein have columnspacers with two and three different sizes respectively, any number ofsizes may be used with the size of the spaces increasing from the upperend to the lower end.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A liquid crystal display device comprising: opposite first and secondsubstrates; and column spacers between the first and second substrates,wherein the column spacers formed on one of the substrates such that theheight of the column spacers differ as position of the column spacersdiffer.
 2. The liquid crystal display device as claimed in claim 1,wherein the column spacers at a lower end of the display device isgreater in height than the column spacers at an upper end of the displaydevice.
 3. The liquid crystal display device according to claim 1,wherein the column spacers have two or more different heights.
 4. Theliquid crystal display device according to claim 1, wherein the columnspacers are formed of photo acryl.
 5. The liquid crystal display deviceaccording to claim 1, wherein the column spacers are formed by an inkjet method.
 6. The liquid crystal display device according to claim 1,wherein the height of the column spacers increases as it goes fartherfrom an upper end to a lower end of the display device.
 7. The liquidcrystal display device according to claim 1, wherein the column spacersin an upper end of the display device have a 3.5 μm height and thecolumn spacers in a lower end of the display device have a 4 to 4.5 μmheight.
 8. The liquid crystal display device according to claim 1,wherein the column spacers in an upper part have a height different fromthe column spacers at a lower end of the display device by less than 1μm. 9-16. (canceled)
 17. A liquid crystal display device comprising:opposite first and second substrates; and column spacers between thefirst and second substrates, wherein the column spacers are formed onone of the substrates using an inkjet method such that the height of thecolumn spacers are substantially uniform.
 18. (canceled)